1. Field of the Invention
The present invention relates to a spatial light modulator (hereinafter abbreviated as an SLM) used in a projection-type display, a holography television, or an optical computer.
2. Description of the Related Art
An optically addressed SLM is positively developed as an essential part of a projection-type display apparatus having a large screen size which is an alternative to a thin film transistor (TFT) liquid crystal panel or a cathode ray tube (CRT).
An SLM performs a light amplification of a two-dimensional image pattern. The SLM includes a photoconductive layer and a light-modulating layer as main components thereof. When image information with low luminance enters the SLM from the side of the photoconductive layer, the electric characteristics of the photoconductive layer are modulated depending on the luminance of the image information. As the result of the modulation, the optical characteristics of the light-modulating layer are modulated. Then, reading light with high luminance enters the SLM from the side of the light-modulating layer, whereby amplified image information can be output.
As the material of the photoconductive layer, CdS, crystalline silicon (Si), amorphous Si (axe2x80x94Si:H), or the like is used. Among them, axe2x80x94Si:H is widely used, since axe2x80x94Si:H has various superior characteristics such as high sensitivity to writing light, a low dark conductivity, wide variations of film formations, and the like. The erasure of the written image is performed without using special erasure pulse light, so that the photoconductive layer is often provided with rectification.
As the material of the light-modulating layer, electrooptic crystals or liquid crystals are used. Among such crystals, surface stabilized ferroelectric liquid crystals (SSFLC; hereinafter referred to simply as PLC) are positively used since the FLC has a faster response (about 100 xcexcsec.) as compared with the conventional liquid crystal of the twisted nematic (TN) type.
It is conventionally known that the FLC has bistability (the binary characteristics). Specifically, the direction of spontaneous polarization of FLC is changed depending on the polarity of the applied electric field, so that the FLC has two different optical states, i.e., ON (or UP) and OFF (or DOWN). The bistability is described in detail in, for example, Appl. Phys. Lett., vol. 36 (1980) pp. 899-901.
In the case where the FLC is used practically in a display or the like, the FLC must be able to display a half-tone state between the ON state and the OFF state. In order to realize the half-tone state, the amount of applied charges is controlled, rather than the externally applied electric field. Since the FLC has the spontaneous polarization which is represented by the spontaneous polarization charge Ps, it is that the aging due to the decomposition of the FLC molecules can be prevented. On the other hand, the driving waveform has the following problems. The voltage of the writing pulse (characterized by a writing period and a writing voltage) 202 is a very high negative voltage, so that the FLC polarization is in verted (electric field switching) even in a condition without writing light. As a result, the contrast is deteriorated. Also, since the duty ratio of the output light is xc2xd at most, this causes a loss of brightness. The driving method using the voltage waveform in FIG. 2(b) was proposed by the inventors of this invention. In the driving method, the obtained duty ratio can substantially be 1. However, also in this method, it is impossible to prevent the contrast deterioration due to the electric field switching during the application of the writing pulse 205.
FIG. 3 shows a driving pulse waveform used for solving the above problems. Such waveforms are used, for example, in SID Digest (1991), pp. 254-256 and U.S. Pat. No. 5,178,445. The driving pulse includes a short erasure pulse 301 with a high voltage and a long writing pulse 302 with a low voltage. In the driving pulse, the absolute value of the voltage of the writing pulse 302 is set to be smaller than the absolute value of the voltage of the erasure pulse 301, so that it is possible to prevent the electric field switching. In addition, since the period of the writing pulse 302 is long, the duty ratio of the reading light can substantially be 1. Thus, the driving method is suitable for the application to a projection-type display or the like. The latter reference specifically mentions the driving voltage conditions for the driving with high contrast. However in both cases, the employed FLC cannot stably have states other than the ON and OFF states. In the former case, the half-tone display is realized by using the above-mentioned multi-domain gray-scale. The latter case uses, as the writing light, pulse light (for example, the emitted light from CRT having phosphors with shorter emission time than the length of one driving period of the SLM).
When the image on a CRT is written on the SLM, the period of the driving pulse voltage signal of the SLM is generally synchronized with one display period of the CRT. FIG. 4 shows the timing chart for driving the CRT and the SLM. In FIG. 4, the timing chart (a) indicates the synchronization pulse voltage of the CRT, and the timing chart (b) indicates the fluorescence intensity from a phosphor on the CRT screen. Also, the timing chart (c) indicates the voltage of a driving pulse of the SLM which is synchronized with one display period of the CRT, and the timing chart (d) indicates the intensity of an output light from the SLM. As is shown in the timing chart (c), a unit driving pulse voltage signal 404 includes an erasure pulse 402 and a reading pulse (characterized by a reading period and a reading voltage) 403, and the signal 404 is synchronized with a synchronization pulse 401 of the CRT display shown in the timing chart (a). The SLM is driven by the driving pulse voltage signal 404. As is shown in the timing chart (b), at a certain timing in the reading period of reading pulse 403 of the driving pulse voltage signal 404, a phosphor of the CRT emits a writing light pulse 405. The light is received by the photoconductive layer of the SLM, and the light-modulating layer of the SLM is switched into the ON state. As a result, as is shown in the timing chart (d), the intensity 406 of the output light from the SLM rises. When an erasure pulse 402 of the next unit driving pulse voltage signal 404 is applied, the light-modulating layer is switched into the OFF state, so that the intensity 406 of the output light from the SLM becomes 0. By repeatedly performing the above operations, the output light from the SLM can be observed. This method has an advantage in that the duty ratio of the reading light (the ratio of the ON state period of the reading light to one driving period) can be increased even in the CRT screen having phosphors which emit fluorescence with a short decay time. In general, in the case where a negative voltage of a large value is applied to the SLM, the SLM may erroneously be switched into the ON state due to the electric field caused by the negative voltage even if the writing light is not incident. However, if the driving signal having a waveform shown in the timing chart (c) is used, such a negative voltage having a large value is not applied to the SLM, so that the light-modulating layer of the SLM cannot be erroneously switched into the ON state. In addition, the deterioration of the contrast ratio of the image which may be caused by such erroneous switching can be prevented. It is also reported that, by varying the fluorescence intensity from a phosphor on the CRT screen, it is possible to realize the half-tone display of the output light of the SLM.
If an SLM is applied to a projection-type display or a holography television, it is necessary that a stable half-tone display with good contrast and with good controllability can be performed.
The conventional SLM could stably have only two optical states as the light-modulating layer, and hence the multi-domain gray-scale has been used for the half-tone display. Therefore in this method, as the resolution of the input image is increased (i.e., the size of each pixel is reduced), the number of domains included in one pixel is decreased, and hence the number of obtainable gray scales is decreased. This causes a problem in that it is difficult to perform the half-tone display.
Since, in the practical driving of the SLM, the charge control is not performed in the range of the applied charge amount for obtaining the intermediate state of FLC (e.g., see U.S. Pat. No. 5,178,445), it is difficult to perform the half-tone display with high contrast.
In addition, the time-averaged value of voltages applied to the FLC is not 0, so that the switching threshold voltage is changed as a function of time due to a drift of ions in the liquid crystal molecules. This causes another problem in that the input/output response characteristics of the light-modulating layer are changed as a result of a long time driving.
In the case where one frame of CRT as the writing means (i.e., one display period) is completely synchronized with the driving period of the SLM, it is difficult to make the brightness uniform over every portion of the SLM. This is described below with reference to FIG. 5. In FIG. 5, (a) is a plan view showing scanning lines 501 and 503 on the CRT screen 505. The timing chart (b) shows the waveform of the driving signal (the driving pulse voltage) of the CRT and the SLM. The timing chart (c) indicates the fluorescence intensity from a pixel 502 of the CRT screen 505, and the timing chart (d) indicates the intensity of output light from a pixel of the SLM corresponding to the pixel 502. The timing chart (e) indicates the fluorescence intensity of a pixel 504 of the CRT screen 505, and the timing chart (f) indicates the intensity of output light from a pixel of the SLM corresponding to the pixel 504 of the CRT screen 505.
In general, the CRT screen 505 is scanned with an electron beam, and light is emitted from the pixels 502 and 504 in the irradiated portion by the electron beam. As a result, the image is displayed on the CRT screen 505. When the electron beam scans the CRT screen 505 from the top to the bottom, the pixel 502 on the scanning line 501 in the upper portion of the CRT emits light at an earlier timing, but the pixel 504 on the scanning line 503 in the lower portion of the CRT emits light at a later timing. As a result, if the driving pulse shown in (b) of FIG. 5 which includes an erasure pulse 506 and a reading pulse 507 is used and the reading period of the reading pulse 507 is set to be substantially equal to one frame of CRT, as a result, the light pulses from the pixels 502 and 504 have waveforms indicated by 508 and 510 shown in the timing charts (c) and (e) in FIG. 5. The intensities of the output light from the pixels of the SLM corresponding to the light pulses 508 and 510 have waveforms indicated by 509 and 511 shown in the timing charts (d) and (f). Even if it is assumed that the writing light pulses 508 and 510 have an equal intensity, the averaged value of the intensity of the output reading light 511 is smaller than the averaged value of the intensity of the output reading light 509 in one-and the same period. Accordingly, a viewer senses that the output light 511 is darker than the output light 509, that is, the upper left portion of the screen 505 is brighter than the lower right portion thereof. For the above-mentioned reasons, the brightness of every portion of the output from the SLM is not uniform.
The spatial light modulator of this invention includes a pair of facing transparent electrodes, and a light-modulating layer and a photoconductive layer provided between the transparent electrodes, wherein the light-modulating layer has different optical states depending on an applied charge amount, the light-modulating layer having: a first optical state when the applied charge amount is a first threshold charge amount or more; a second optical state when the applied charge amount is a second threshold charge amount or less; and a spatially uniform intermediate state between the first optical state and the second optical state depending on the applied charge amount.
In one embodiment of the invention, the photoconductive layer has rectification, and generates, when the photoconductive layer is in a reversed bias condition, a photoelectric current having a magnitude depending on an intensity of writing light incident on the photoconductive layer.
In another embodiment of the invention, the light-modulating layer includes a ferroelectric liquid crystal layer which is sandwiched by two alignment films.
In another embodiment of the invention, wherein a specific resistance of the alignment films is in the range of 108 xcexa9xc2x7cm to 1011 xcexa9xc2x7cm.
According to another aspect of the invention, a driving method for a spatial light modulator is provided. In the method, the spatial light modulator includes a pair of facing transparent electrodes, and a light-modulating layer and a photoconductive layer provided between the transparent electrodes, the light-modulating layer having different optical states depending on an applied charge amount, the light-modulating layer having: a first optical state when the applied charge amount is a first threshold charge amount or more; a second optical state when the applied charge amount is a second threshold charge amount or less; and a spatially uniform intermediate state between the first optical state and the second optical state depending on the applied charge amount, wherein the photoconductive layer has rectification, and generates, when the photoconductive layer is in a reversed bias condition, a photoelectric current having a magnitude depending on an intensity of writing light incident on the photoconductive layer, wherein one period of a waveform of a driving voltage includes an erasure period in which the photoconductive layer is in a forward bias condition and a charge amount larger than the first threshold charge amount is applied, and a writing period in which the photoconductive layer is in a reversed bias condition so as to generate a photoelectric current having a magnitude depending on an intensity of writing light, and wherein the method includes a step of applying the driving voltage to the two transparent electrodes, so that in the writing period, the applied charge amount to the light-modulating layer is kept in the range of the first threshold charge amount or more when the intensity of the writing light is a first threshold light intensity or lower, and the applied charge amount to the light-modulating layer is reduced to the second threshold charge amount or less when the intensity of the writing light is a second threshold light intensity of higher.
In one embodiment of the invention, the light-modulating layer includes a ferroelectric liquid crystal layer which is sandwiched by two alignment films, and wherein conditions of:
xe2x88x92Vthxe2x89xa6(CfVe+CaVw)/(Cf+Ca)xe2x88x92Vd, and
Vwxe2x88x92Vdxe2x89xa6xe2x88x92Vth
are satisfied, where Ve denotes the maximum value of the driving voltage in the erasure period, Vw denotes the minimum value of the driving voltage in the writing period, Cf denotes a capacitance of the ferroelectric liquid crystal layer without polarization inversion of the ferroelectric liquid crystal layer, Ca denotes a capacitance of the photoconductive layer, Vd denotes a diffusion potential of the photoconductive layer, and xe2x88x92Vth denotes a threshold voltage of the ferroelectric liquid crystal layer.
In another embodiment of the invention, the driving voltage Ve in the erasure period and the driving voltage Vw in the writing period are respectively in the ranges of:
1 Vxe2x89xa6Vexe2x89xa640 V, and
xe2x88x9220 Vxe2x89xa6Vwxe2x89xa64 V.
According to another aspect of the invention, a driving method for a spatial light modulator is provided. In the driving method, the spatial light modulator includes a pair of facing transparent electrodes, and a light-modulating layer and a photoconductive layer provided between the transparent electrodes, the light-modulating layer having different optical states depending on an applied charge amount, the light-modulating layer having: a first optical state when the applied charge amount is a first threshold charge amount or more; a second optical state when the applied charge amount is a second threshold charge amount or less; and a spatially uniform intermediate state between the first optical state and the second optical state depending on the applied charge amount, and wherein the driving method includes the steps of: applying a driving voltage to the two transparent electrodes; irradiating the spatial light modulator with writing light; measuring an output light intensity of the spatial light modulator with respect to at least one writing light intensity; detecting a change of the output light intensity as a function of time; and performing a feedback to at least one of the writing light intensity and the driving voltage, in accordance with the detected change of the output light intensity.
In one embodiment of the invention, the photoconductive layer has rectification, and generates, when the photoconductive layer is in a reversed bias condition, a photoelectric current having a magnitude depending on an intensity of writing light incident on the photoconductive layer, wherein one period of a waveform of a driving voltage includes an erasure period in which the photoconductive layer is in a forward bias condition and a charge amount larger than the first threshold charge amount is applied, and a writing period in which the photoconductive layer is in a reversed bias condition so as to generate a photoelectric current having a magnitude depending on an intensity of writing light, and wherein the driving voltage, in the writing period, keeps the applied charge amount to the light-modulating layer in the range of the first threshold charge amount or more when the intensity of the writing light is a first threshold light intensity or lowers and reduces the applied charge amount to the light-modulating layer to the second threshold charge amount or less when the intensity of the writing light is a second threshold light intensity of higher.
In another embodiment of the invention, the light-modulating layer includes a ferroelectric liquid crystal layer which is sandwiched by two alignment films, wherein conditions of:
xe2x88x92Vthxe2x89xa6(CfVe+CaVw)/(Cf+Ca)xe2x88x92Vd, and
Vwxe2x88x92Vdxe2x89xa6xe2x88x92Vth
are satisfied, where Ve denotes the maximum value of the driving voltage in the erasure period, Vw denotes the minimum value of the driving voltage in the writing period, Cf denotes a capacitance of the ferroelectric liquid crystal layer without polarization inversion of the ferroelectric liquid crystal layer, Ca denotes a capacitance of the photoconductive layer, Vd denotes a diffusion potential of the photoconductive layer, and xe2x88x92Vth denotes a threshold voltage of the ferroelectric liquid crystal layer, wherein the driving method includes a step of keeping values of L1 and Ls constant by changing at least one of Ve, Vw, Tw and xcfx84, the values of L1 and L2 being defined by
L1=(hxcexd/xcex7e)(Cf+Ca)(Vf0+Vth)/Twxcfx84,
Ls=(hxcexd/xcex7e)(2Ps/Twxcfx84), and
Vf0=(CfVe+CaVw)/(Cf+Ca)xe2x88x92Vd,
where Tw denotes a width of the writing period, xcfx84 denotes a ratio (utilization efficiency) of an intensity of light actually incident on the photoconductive layer to the intensity of the writing light, hxcexd denotes a photon energy of the writing light, xcex7 denotes a quantum efficiency of the photoconductive layer, and e denotes an electron charge.
In another embodiment of the invention, the driving method further includes the steps of: measuring changes dYA and dYB in output light intensities of the spatial light modulator as a function of time with respect to two different writing light intensities L=LA and L=LB; obtaining changes dL1 and dLs of L1 and L2 by using equations of
dL1=[(∂Y/∂Ls)L=LBxc2x7dYAxe2x88x92(∂Y/∂Ls)L=LAxc2x7dYB]/xcex94,
dLs=[xe2x88x92(∂Y/∂L1)L=LBxc2x7dYA+(∂Y/∂L1)L=LAxc2x7dYB]/xcex94,
and
xcex94=(∂Y/∂L1)L=LAxc2x7(∂Y/∂Ls)L=LBxe2x88x92(∂Y/∂Ls)L=LAxe2x88x92(∂Y/∂L1)L=LB; and
changing Ve, Vw, Tw and xcfx84 by amounts equal to dVe, dVw, dTw and dxcfx84, respectively, so as to satisfy relationships of
xe2x88x92dL1=(∂L1/∂Ve)dVe+(∂L1/∂Vw)dVw+(∂L1/∂Tw)dTw+(∂L1/∂xcfx84)dxcfx84, and
xe2x88x92dLs=(∂Ls/∂Tw)dTw+(∂Ls/∂xcfx84)dxcfx84.
In another embodiment of the invention, the driving method further includes the steps of: measuring changes dYA and dYB in output light intensities of the spatial light modulator as a function of time with respect to two different writing light intensities L=LA and Lxe2x88x92LB; obtaining changes dL1 and dLs of L1 and Ls by using equations of
dL1=[(∂Y/∂Ls)L=LBxc2x7dYAxe2x88x92(∂Y/∂Ls)L=LAxc2x7dYB]/xcex94,
dLs=[xe2x88x92(∂Y/∂L1)L=LBxc2x7dYA+(∂Y/∂L1)L=LAxc2x7dYB]/xcex94,
and
xcex94=(∂Y/∂L1)L=LAxc2x7(∂Y/∂Ls)L=LBxe2x88x92(∂Y/∂Ls)L=LAxc2x7(∂Y/∂L1)L=LB; and
changing Ve, Vw, Tw and xcfx84 by amounts equal to dVe, dVw, dTw and dxcfx84, respectively, so as to satisfy relationships of
dve=0,
dVw[(∂L1/∂Tw)dLsxe2x88x92(∂Ls/∂Tw)dL1]/[(∂L1/∂Vw) (∂Ls/∂Tw)],
dTw=xe2x88x92(∂Ls/∂Tw)xe2x88x921dLs, and
dxcfx84=0.
In another embodiment of the invention, the driving method further includes the steps of: measuring changes dYA and dYB in output light intensities of the spatial light modulator as a function of time with respect to two different writing light intensities L=LA and Lxe2x88x92LB; obtaining changes dL1 and dLs of L1 and Ls by using equations of
dL=[(∂Y/∂Ls)L=LBxc2x7dYAxe2x88x92(∂Y/∂Ls)L=LAxc2x7dYB]/xcex94,
dLs=[xe2x88x92(∂Y/∂L1)L=LBxc2x7dYA+(∂Y/∂L1)L=LAxc2x7dYB]/xcex94,
and
xcex94=(∂Y/∂L1)L=LAxc2x7(∂Y/∂Ls)L=LBxe2x88x92(∂Y/∂Ls)L=LAxc2x7(∂Y/∂L1)L=LB; and
changing Ve, Vw, Tw and xcfx84 by amounts equal to dVe, dVw, dTw and dxcfx84, respectively, so as to satisfy relationships of
dVe=0,
dVwxe2x88x92[(∂L1/xcfx84)dLsxe2x88x92(∂Ls/xcfx84)dL1]/[(∂L1/∂Vw)(∂Ls/∂xcfx84)],
dTw=0, and
dxcfx84xe2x88x92(∂Ls/∂xcfx84)xe2x88x921dLs.
In another embodiment of the invention, the driving method includes a step of measuring output light intensities of the spatial light modulator with respect to three or more different writing light intensities.
In another embodiment of the invention, the driving method further includes the steps of: irradiating the spatial light modulator with the writing light via an image presentation portion and an intensity modulating portion which modulates an intensity of an image presented on the image presentation portion; and performing a feedback to a transmittance of the intensity modulating portion, in accordance with a change of the output light intensity as a function of time.
In another embodiment of the invention, the writing light is generated from a CRT, and the driving method comprises a step of performing a feedback to an electron beam current value of the CRT in accordance with the change of the output light intensity as a function of time.
In another embodiment of the invention, the measurement of the output light intensities of the spatial light modulator is performed directly after the output side of the spatial light modulator.
In another embodiment of the invention, one period of the driving voltage waveform for driving the spatial light modulator is shorter than one display period of an image formed by the writing light.
In another embodiment of the invention, a ratio of one display period of an image formed by the writing light to one period of the driving voltage is in the range of 1.5 to 1000.
In another embodiment of the invention, the output light intensity of the spatial light modulator with respect to the writing light intensities with the first threshold light intensity or lower of the spatial light modulator is substantially 0, the output light intensity with respect to the writing light intensities between the first threshold light intensity and the second threshold light intensity is increased as the writing light intensity is increased, and the output light intensity with respect to the writing light intensities which exceeds the second threshold light intensity has substantially no dependence on the writing light intensity.
In another embodiment of the invention, the writing light intensity to the spatial light modulator is substantially monotonously decreased as a function of time in one display period of an image formed by the writing light, the maximum value of the writing light intensity directly prior to the end of the display period is the second threshold light intensity or higher, and the maximum value decays to be the first threshold light intensity or lower in a period in which an image is rewritten by the writing light.
In another embodiment of the invention, the writing light is generated from a CRT.
In another embodiment of the invention, the driving voltage applied in the erasure period is in the range of +2 V to +30 V by regarding a direction in which the photoconductive layer is forward-biased as a positive, and the driving voltage applied in the writing period is in the range of xe2x88x9230 V to +2 V.
In another embodiment of the invention, one period of the driving voltage is constituted by a sequence of the erasure period, a first low voltage period, the writing period, and a second low voltage period.
In another embodiment of the invention, the second low voltage period is longer than the first low voltage period.
In another embodiment of the invention, the driving voltage applied in the erasure period is in the range of +2 V to +30 V by regarding a direction in which the photoconductive layer is forward-biased as a positive, the driving voltage applied in the writing period is in the range of xe2x88x9230 V to xe2x88x922 V, and the driving voltage applied in the second low voltage period is in the range of xe2x88x922 V to +2 V.
In another embodiment of the invention, the ferroelectric liquid crystal layer and the photoconductive layer are electrically in contact with each other via a metal reflection film which is divided and separated into minute portions.
According to another aspect of the invention, a spatial light modulating apparatus is provided. The spatial light modulating apparatus includes: a spatial light modulator including a light-modulating layer and a photoconductive layer provided between two facing transparent electrodes, the light-modulating layer having different optical states depending on an applied charge amount, the light-modulating layer having: a first optical state when the applied charge amount is a first threshold charge amount or more; a second optical state when the applied charge amount is a second threshold charge amount or less; and a spatially uniform intermediate state between the first optical state and the second optical state depending on the applied charge amount; means for applying a driving voltage to the two transparent electrodes; means for irradiating the spatial light modulator with writing light; means for measuring an output light intensity of the spatial light modulator with respect to at least one writing light intensity; means for detecting a change of the output light intensity as a function of time with respect to the at least one writing light intensity; and means for performing a feedback to at least one of the writing light intensity and the driving voltage in accordance with the detected change of the output light intensity.
According to another aspect of the invention, a liquid crystal device is provided. The liquid crystal device includes a ferroelectric liquid crystal layer sandwiched by two opposing alignment films, and means for applying charges to the ferroelectric liquid crystal layer, wherein a specific resistance of the alignment films is in the range of 108 xcexa9xc2x7cm to 1011 xcexa9xc2x7cm.
Thus, the invention described herein makes possible the advantages of (1) providing a spatial light modulator which can realize a half-tone display with high contrast, (2) providing a spatial light modulator which is stable for a long use of time, (3) providing a spatial light modulator which can display an image with uniform brightness, and (4) a driving method for such a spatial light modulator.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.